A nonwoven fabric has been used as a web of fibers useful for filters (such as air filters and bag filters), separators for cells (such as nickel-hydrogen cells, nickel-cadmium cells and lithium ion cells), reinforcing materials of substrates for printed circuit boards, reinforcing materials of electrolyte membranes, etc.
As materials for the nonwoven fabric, general-purpose materials such as polypropylene, polyester or polyamide have been used. However, in recent years, a fluororesin which is excellent in heat resistance, chemical resistance, non-stickiness, etc. has been used as a material for a separator of a cell, a high performance filter for air cleaning or a filter for filtration of a chemical in the field of semiconductors, a bag filter for anti-pollution measures, etc.
As the nonwoven fabrics made of a fluororesin, the following have been known.
(1) A nonwoven fabric obtained by stretching polytetrafluoroethylene, cutting it and forming it into fibers, followed by a water jet method or a needle punching method to make the fibers cross each other.
(2) A nonwoven fabric obtained by meltblowing an ethylene/chlorotrifluoroethylene copolymer (Patent Document 1).
(3) A nonwoven fabric obtained by meltblowing a tetrafluoroethylene copolymer (Patent Document 2).
(4) A nonwoven fabric obtained by meltblowing an ethylene/tetrafluoroethylene copolymer (hereinafter referred to as ETFE) (Patent Document 3).
However, the strength of the nonwoven fabric (1) is insufficient, since fibers are not mutually fused.
The nonwoven fabric (2) is insufficient in the chemical resistance, the water repellency, the stain proofing property, the mold release property, etc.
The tetrafluoroethylene copolymer as the material of the nonwoven fabric (3) has a low elastic modulus at around room temperature at which the nonwoven fabric is to be used, and accordingly the nonwoven fabric is insufficient in the mechanical strength.
On the other hand, ETFE which is the material of the nonwoven fabric (4) has a high elastic modulus at around room temperature at which the nonwoven fabric is to be used, and has excellent heat resistance, chemical resistance and non-stickiness. However, in a case where a nonwoven fabric is produced by meltblowing ETFE for general forming, since the melt viscosity of ETFE is high, the average fiber diameter of the fibers that make up the nonwoven fabric tends to be large. As a result, if such a nonwoven fabric is used to reinforce an electrolyte membrane, the electrolyte membrane tends to be thick, and the resistance will be increased. Further, when such a nonwoven fabric is used as a filter for removing fine particles, the maximum pore diameter tends to be large, whereby the fine particle removing performance will be insufficient.